Chemical modification of
Rhodospirillum rubrum chromatophores by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) results in inactivation of photophosphorylation, Mg
2+-ATPase, oxidative phosphorylation and ATP-driven transhydrogenase, with apparent first-order kinetics. Other energy-linked reactions such as light-driven transhydrogenase and light-dependent proton uptake were insensitive to NBD-Cl. The Ca
2+-ATPase activity of the soluble coupling factor from chromatophores (
R. rubrum F
1) was inactivated by NBD-Cl with kinetics resembling those described for Mg
2+-ATPase and photophosphorylation activities of chromatophores. Both NBD-chromatophores and NBD-
R. rubrum F
1 fully recovered their activities when subjected to thiolysis by dithioerythritol. Phosphoryl transfer reactions of chromatophores and Ca
2+-ATPase activity of
R. rubrum F
1 were fully protected by 5 mM P
i against modification by NBD-Cl. ADP or ATP afforded partial protection. Analysis of the protection of Ca
2+-ATPase activity by P
i indicated that NBD-Cl and P
i are mutually exclusive ligands. Spectroscopic studies revealed that tyrosine and sulfhydryl residues in
R. rubrum F
1 underwent modification by NBD-Cl. However, the inactivation was only related to the modification of tyrosine groups.
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